docs: use latest node in travis guide (conventional-changelog#871)

The original Travis config does not use high enough version of node so the commitlint does not work. Updated docs accordingly

Author: vojtechruz

docs/guides-ci-setup.md

@@ -14,6 +14,8 @@ npm install --save-dev @commitlint/travis-cli
 ```yml
 # travis.yml
 language: node_js
+node_js:
+  - node
 script:
   - commitlint-travis
 ```